1. Field of the Invention
The present invention relates to a bearing assembly and a method for manufacturing the same, and more particularly, to a bearing assembly in which a fiber reinforced composite (FRC) is used to secure mechanical strength as well as to reduce a friction coefficient and stress concentration, thereby improving reliability, and a method for manufacturing the same.
2. Description of the Background Art
A hemispherical bearing is a mechanical element that supports a spherical journal and is used in a variety of machines and apparatuses. The spherical journal and the hemispherical bearing are used as a ball joint for connecting two components to allow free movement of the two components. In the case of a ball joint, the hemispherical bearing is called a “housing socket” and the spherical journal is called a “ball stud.”
The structure of a bearing assembly into which a conventional spherical journal and a conventional hemispherical bearing are combined will be described with reference to FIG. 1. The hemispherical bearing 10 has a generally hemispherical bearing surface 12, and the spherical journal 20 is received in the bearing surface 12 which establishes a spherical pair with the spherical journal 20. A connecting rod 22 that transmits a load is connected to a side of an outer surface of the spherical journal 20, and the hemispherical bearing 10 is mounted in a recess 32 of a piston 30. The connecting rod 22 moves in the direction which a load is applied and simultaneously rotates within a certain angular range by means of the spherical pairing of the spherical journal 20 and the hemispherical bearing 10, and the load applied to the connecting rod 22 is transmitted to the piston 30.
The conventional hemispherical bearing 10 has been used in a suspension of a caterpillar vehicle with high mobility. According to the kind of caterpillar vehicle and the installation location of the hemispherical bearing 10, an average surface pressure of 200 MPa or more is applied on the bearing surface 12. Therefore, the hemispherical bearing 10 should be made of a material which can withstand high compressive force. Further, since it is usually difficult to continuously supply a lubricant oil due to characteristics of the installation of the hemispherical bearing 10, it is required that the material for the hemispherical bearing 10 have high compressive strength as well as oil-free lubrication and wear-resistant properties.
Accordingly, a strand prepreg made by impregnating reinforcement fibers such as carbon fibers or graphite fibers in a thermoplastic resin has been used as the material for the hemispherical bearing 10. In such a strand prepreg, reinforcement fibers are arranged in several directions or in random directions without directivity. The strand prepreg is subjected to cutting and hot compression molding to manufacture the hemispherical bearing 10. Further, the hemispherical bearing 10 may be made of a thermoplastic composite such as polyetheretherketone (PEEK) reinforced with carbon fiber fabrics.
The bearing surface 12 of the hemispherical bearing 10 is conventionally made by cutting a thick plate that is formed by laminating a fiber reinforced thermoplastic composite into multiple layers and performing hot compression molding thereto. However, such a conventional technique has a problem in that it is difficult to perform cutting due to severe tool wear. Moreover, the conventional technique has a problem in that a processing rate cannot be increased due to damage to the strand prepreg, resulting in increased production costs. In addition, the conventional technique has a problem in that an allowable load is lowered due to cracks generated in the hemispherical bearing 10.
Meanwhile, when parts such as the hemispherical bearing 10 and the piston 30 are assembled or disassembled, a hermetically sealed space is formed between the hemispherical bearing 10 and the piston 30, which makes the assembly and disassembly of the hemispherical bearing 10 difficult. To overcome the aforementioned problems, in the prior art, an air vent hole 14 that penetrates the center of the hemispherical bearing 10 is formed to facilitate the assembly and disassembly of the hemispherical bearing 10 and the piston 30.
In addition, the reinforcement fiber exhibits anisotropy in which friction coefficients in longitudinal and vertical directions differ from each other according to the kind thereof. For example, carbon fiber is a material with high anisotropy in which a friction coefficient is low in a longitudinal direction. The arrangement orientation of the reinforcement fibers is not constant on the bearing surface 12 of the hemispherical bearing 10 manufactured through the hot compression molding of the strand prepreg. Accordingly, in the conventional hemispherical bearing 10, the moving direction of the spherical journal 20 cannot be coincident with the orientation of the reinforcement fibers, thereby causing a disadvantage in that the anisotropy of the reinforcement fibers in view of their wear properties cannot be sufficiently utilized.
Further, the formation of the air vent hole 14 at the center of the bearing surface 12 with greatest stress concentration has a potential problem in that a crack may be easily created from the periphery of the air vent hole 14. The growth of such a crack causes breakage of the hemispherical bearing 10, greatly reducing its life and lowering its reliability. In addition, there is a disadvantage in that hard particles or debris created due to wear of the spherical journal 20 and the hemispherical bearing 10 aggravate the wear phenomenon.